When switching current flowing to an inductive load it is not possible to turn off the current instantaneously because that would induce an infinite voltage across the inductor. Nevertheless, when current is switched off, the voltage across the inductor rises in an attempt to keep current flowing. Eventually, the voltage may reach a level that causes current flow by arcing across the switch. If the switch is a semiconductor device, a voltage exceeding its maximum collector to emitter voltage will cause breakdown, and hence destruction, of the device.
The device breakdown problem is solved in a dc circuit by using a flyback diode to clamp transient voltages and reduce switching losses. The flyback diode is reverse biased when the switch is on and goes into conduction when the switch is turned off, clamping the inductor voltage to one diode drop.
If the inductive load is driven from an ac source, clamping transient voltages is much more difficult since it is impossible to construct a bilateral flyback diode; that is, such a device would conduct on each half cycle when the switch is closed, and thus essentially act simply as a conductor. A transient suppressor such as a metal-oxide varistor, which behaves like a bilateral zener diode, may be used across the inductive load. Alternatively, an RC snubber network may be used across the inductive load. Another possible solution would be to reduce the switching speed so that the product of the inductance (L) and the rate of change of current (di/dt) is a practical number, although current flow for this longer period of time would increase the total switching losses. The use of a snubber or a varistor may be undesirable in applications where it is intended to use the current flowing from the inductor rather than dissipate the current. For example, the light output of fluorescent lamps may be dimmed by placing a switch in the power line and chopping the input waveform to the inductive ballast. The energy stored in the ballast primary may be transferred to the fluorescent lamps (i.e. through the ballast secondary) when the series power line switch is open by clamping the ballast primary voltage.
One known technique for clamping an inductive ac load is to shunt the inductive load with a controlled switch such that when the power line switch is open the shunt switch is closed, and vice versa. As a result, the control complexity of ac circuits is doubled since each switch is usually a unipolar switch connected to a bridge rectifier and must be transformer controlled or photovoltaically controlled. Furthermore, the opening times of the switches must be set long enough to keep L di/dt, the voltage rise across the inductor, within the device ratings of the circuit components.
Simultaneous switching of the shunt switch has the further disadvantage that phase differences occurring in the load as the series switch chops the line voltage at different rates or times are not accounted for. Thus, further complexity must be added to account for the phase differences and thereby hold voltage to less than an excessive level.
Yet another possible solution to this problem is to connect a sidac device across the inductive load. The sidac is a high voltage bilateral trigger device available from Motorola, Inc. The sidac clamps load voltage by breaking over when load voltage exceeds the sidac's trigger voltage. However, at sufficiently high rates of change of voltage the sidac fails to clear at current zero, shorting the line and destroying the line switch and itself.